Modern security and surveillance systems, such as those providing intruder or burglar alarms, typically comprise a central control panel connected to a plurality of peripherals such as video cameras, microphones, pressure sensors, temperature sensors, movement detectors, etc. When a possible threat is detected by any of the peripherals, said peripheral alerts the central control panel, which in turn transmits an alert signal to a remote centralized back-end service. Communications between peripherals and control panel, and between control panel and back-end services can be performed either through wired or through wireless communication systems. However, wired communications can be easily tampered, effectively disengaging the security system. Wireless communications are hence regarded as being more secure, but they can also be subjected to attacks through intentional jamming. When this occurs, the wireless communication channel is actively interfered, reducing the signal to noise ratio of the transmitted signals and rendering communication impossible. Although less likely, a similar scenario can occur as a result of unintentional interferences from nearby radio frequency systems.
In order to establish defenses against this kind of accidental or active jamming, several alternatives have been proposed in the state of the art. For example, patent document GB-2457102-A discloses an intruder detection system where the alarm signal is transmitted wirelessly. In case of jamming, the interference is detected by a sensor, and the transmission frequency is modified to avoid the jammed frequency range. However, modifying the frequency may not be enough to avoid the jamming. If the intruder manages to completely jam all the frequency band within which said transmission frequency can be tuned, the alarm signal will still not be received.
Patent document US-2014/0369452-A1 presents an alternative approach based on a multi-channel communication link between the transmitter and the receiver of a global navigation satellite system (GNSS) network. When jamming is detected, the number of channels used for data transmission is progressively reduced, increasing jamming resilience. Nevertheless, this approach could not be straightforwardly applied to wireless communication networks used in security systems such as Industrial Scientific and Medical (ISM) radio networks and Global System for Mobile Communications (GSM) networks. Furthermore, the maximum achievable jamming protection is limited by the channel bandwidth of the architecture of the network used for the communication link, which could prove insufficient in some scenarios.
Finally, patent document U.S. Pat. No. 4,117,405-A1 discloses an alarm transmission system relying on an ultra narrow band network (UNB), given its inherent resistance against jammers. The system comprises a plurality of alarm stations, each of them locked to a finely tuned local oscillator. A distinctive local oscillator frequency is assigned to each alarm station. However, although some jamming resistance is incorporated, the actual presence of jamming is neither checked nor notified to the back-end service, hence giving the intruders more freedom during their tampering attempt. Furthermore, by using a fixed frequency in each station, the system is more predictable and easily tampered, and it requires significant resources to be employed in calibration and frequency drift corrections. Finally, UNB networks are devised for multi-point to point services in which each element rarely needs to access the communication medium. Using the UNB by default in all communications between elements of the security system limits the total traffic and the number of deployable alarm stations.
Therefore, there is still the need in the state of the art of a jamming detection technique that effectively protects security systems against aggressive jamming of their wireless communications. Furthermore, it is still desirable that said jamming detection technique does not limit the number of elements connected to the back-end service or the data load among them when no jamming is present, and that it presents a reduced energy consumption for increased autonomy, and therefore, a more prolonged and reliable protection.